Swanson J A, Mulligan J T, Long S R
Department of Biological Sciences, Stanford University, California 94305-5020.
Genetics. 1993 Jun;134(2):435-44. doi: 10.1093/genetics/134.2.435.
The early steps of symbiotic nodule formation by Rhizobium on plants require coordinate expression of several nod gene operons, which is accomplished by the activating protein NodD. Three different NodD proteins are encoded by Sym plasmid genes in Rhizobium meliloti, the alfalfa symbiont. NodD1 and NodD2 activate nod operons when Rhizobium is exposed to host plant inducers. The third, NodD3, is an inducer-independent activator of nod operons. We previously observed that nodD3 carried on a multicopy plasmid required another closely linked gene, syrM, for constitutive nod operon expression. Here, we show that syrM activates expression of the nodD3 gene, and that nodD3 activates expression of syrM. The two genes constitute a self-amplifying positive regulatory circuit in both cultured Rhizobium and cells within the symbiotic nodule. We find little effect of plant inducers on the circuit or on expression of nodD3 carried on pSyma. This regulatory circuit may be important for regulation of nod genes within the developing nodule.
根瘤菌在植物上形成共生根瘤的早期步骤需要几个结瘤基因操纵子的协同表达，这是由激活蛋白NodD完成的。苜蓿共生菌苜蓿根瘤菌的共生质粒基因编码三种不同的NodD蛋白。当根瘤菌暴露于宿主植物诱导物时，NodD1和NodD2激活结瘤操纵子。第三种NodD3是结瘤操纵子的独立于诱导物的激活剂。我们之前观察到，携带在多拷贝质粒上的nodD3需要另一个紧密连锁的基因syrM才能组成型表达结瘤操纵子。在这里，我们表明syrM激活nodD3基因的表达，并且nodD3激活syrM的表达。这两个基因在培养的根瘤菌和共生根瘤内的细胞中构成一个自我放大的正调控回路。我们发现植物诱导物对该回路或pSyma上携带的nodD3的表达几乎没有影响。这种调控回路可能对发育中的根瘤内结瘤基因的调控很重要。
